When a project export completes, it removes everything in
`Project#import_export_shared.archive_path`, which can erase files
needed for another ongoing project export. This is problematic for
custom templates, which exports an existing project to get the most
recent changes and imports that archive to another project.

To avoid this from happening, we generate a random unique subpath in the
shared temporary directory so that multiple exports can work at the same
time.

Previously the path structure was as follows:

1. Project export files stored in:
   /shared/tmp/project_exports/namespace/project/:random
2. Project export .tar.gz files stored in:
   /shared/tmp/project_exports/namespace/project
3. Project export lock file:
   /shared/tmp/project_exports/namespace/project/.after_export_action

Now:

1. Project export files stored in:
   /shared/tmp/project_exports/namespace/project/:randomA/:randomB
2. Project export .tar.gz files stored in:
   /shared/tmp/project_exports/namespace/project/:randomA
3. Project export lock files stored in:
   /shared/tmp/project_exports/namespace/project/locks

The .tar.gz files are also now cleaned up in the AfterExportStrategy.

Also, ensure import/export path cleanup always happens.

A failure to update the database or object storage shouldn't
block us from cleaning up stale directories. This is especially
important to clear out stale lock file and archive paths.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/14716